The present invention relates to a phase change type optical recording medium, and a method of overwriting the same.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting or overwriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with a laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reproducing. Optical recording media of the phase change type are of great interest since the optical system of the drive unit used for their operation is simpler than that for magneto-optical recording media.
Most optical recording media of the phase change type use Ge-Sb-Te base or chalcogenide materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
When information is recorded on a phase change type optical recording medium, the recording layer is irradiated with laser light of power (recording power) high enough to bring the recording layer to a temperature higher than the melting point thereof. The recording layer is melted at spots with the recording power applied thereon, and then quickly cooled so that recorded marks of amorphous nature can be formed. When the recorded marks are erased, on the other hand, the recording layer is irradiated with laser light having such a relatively low power (erase power) as to bring the recording layer to a temperature higher than the crystallization temperature thereof but lower than the melting point thereof. The recorded marks with the erase power applied thereon go back to the crystalline state because they are slowly cooled down after heated to the temperature higher than their crystallization temperature. With the phase change type optical recording medium, it is thus possible to modulate the intensity of a single light beam for overwriting.
In the phase change type optical recording medium, dielectric layers are formed on both sides of the recording layer. Requirements for these dielectric layers are that:
(1) they can protect the recording layer or substrate against thermal hysteresis due to laser light irradiation, PA1 (2) they can amplify reproducing signals making use of the interference effect of light reflected at the interface of each layer, and PA1 (3) their thermal conductivity, etc. can be properly regulated to control record/erase characteristics. PA1 after at least 10,000 overwriting cycles, a reflectance difference between a recorded mark and an erase area is at least 80% of said reflectance difference in an initial overwriting cycle, with a jitter of up to 13%. PA1 an erase power margin after at least 10,000 overwriting cycles is at least 30% of an erase power margin in an initial overwriting cycle, and PA1 a value obtained by subtracting an optimum erase power after at least 10,000 overwriting cycles from an optimum erase power in an initial overwriting cycle is up to 1 mW, or up to 20% with respect to the optimum erase power in the initial overwriting cycle. PA1 said first dielectric layer, and said second dielectric layer contain zinc sulfide and silicon oxide as a main component, while said first dielectric layer comprises a dielectric lamina 1a on a substrate side thereof and a dielectric lamina 1b on a recording layer side thereof, PA1 said first dielectric lamina 1a has a silicon oxide content or SiO.sub.2 /(ZnS+SiO.sub.2) of 2 mol % to less than 40 mol %, said dielectric lamina 1b has a silicon oxide content of 40 mol % to 100 mol % inclusive, and said second dielectric layer has a silicon oxide content of 2 mol % to 50 mol % inclusive, with the proviso that zinc sulfide and silicon oxide are calculated as ZnS and SiO.sub.2, respectively, and PA1 said second dielectric layer has a thickness of 10 nm to 35 nm inclusive. PA1 a value obtained by subtracting an optimum erase power after at least 10,000 overwriting cycles from an optimum erase power in an initial overwriting cycle is up to 1 mW, or up to 20% with respect to the optimum erase power in the initial overwriting cycle. PA1 said first dielectric layer, and said second dielectric layer contain zinc sulfide and silicon oxide as a main component, while said first dielectric layer comprises a dielectric lamina 1a on a substrate side thereof and a dielectric lamina 1b on a recording layer side thereof and said second dielectric layer comprises a dielectric lamina 2a on a recording layer side thereof and a dielectric lamina 2b on a reflective layer side thereof, PA1 said first dielectric lamina 1a has a silicon oxide content or SiO.sub.2 /(ZnS+SiO.sub.2) of 2 mol % to less than 40 mol %, said dielectric lamina 1b has a silicon oxide content of 40 mol % to 100 mol % inclusive, said dielectric lamina 2a has a silicon oxide content of 2 mol % to 50 mol % inclusive, and said dielectric lamina 2b has a silicon oxide content that is 40 mol % to 100 mol % inclusive and higher than that of said dielectric lamina 2c, with the proviso that zinc sulfide and silicon oxide are calculated as ZnS and SiO.sub.2, respectively, and PA1 said second dielectric layer has a thickness of 10 nm to 35 nm inclusive. PA1 a value obtained by subtracting an optimum erase power after at least 10,000 overwriting cycles from an optimum erase power in an initial overwriting cycle is up to 1 mW, or up to 20% with respect to the optimum erase power in the initial overwriting cycle. PA1 a power level of a laser beam used for overwriting involves three levels, P.sub.p that is peak power, P.sub.B1 that is bias power 1 lower than P.sub.p, and P.sub.B2 that is bias power 2 lower than P.sub.B1, and PA1 a laser beam for forming a recorded mark is pulse modulated in such a way that said peak power is given by P.sub.p and bottom power is given by P.sub.B2, and after final pulse irradiation, said power level goes down to P.sub.B2, and then goes back to P.sub.B1 that is an erase power level.
For a dielectric layer meeting such requirements, a dielectric layer composed mainly of ZnS having a high refractive index is often used. For instance, JP-A 63-103453 discloses an optical information recording medium having a dielectric layer comprising a mixture of ZnS and SiO.sub.2. The publication states that among the advantages of the invention there are an increase in the sensitivity to irradiation power upon recording, and an increase in the number of repetition of write/read cycles to which the dielectric material is exposed, and goes on that the sensitivity increase is due to the optimization of the heat constant of the dielectric layer, and the increase in the number of repetition of write/read cycles is due to a drop of the laser power required for recording and erasing. The publication states that the preferable SiO.sub.2 /(ZnS+SiO.sub.2) ratio is 10 to 30 mol % because the laser energy required for recording and erasure then becomes smallest.
With a phase change type optical recording medium comprising a recording layer constructed of a Ge-Sb-Te base material and dielectric layers on both sides thereof, each composed mainly of ZnS, however, the number of overwritable cycles is limited to several thousand cycles or so because of C/N drops due to repetition of overwriting. A possible leading reason for the C/N drops due to overwriting could be a composition change of the recording layer caused as by the diffusion of elements between the recording layer and the dielectric layers adjacent thereto.
JP-A 2-177141 proposes to reduce reactions between a recording layer and dielectric layers. An optical information recording medium disclosed therein comprises a recording layer for absorbing light to write information thereon and erase the information therefrom, and a protective layer located on at least one side of the recording layer, said protective layer composed primarily of a metal chalcogenide and a compound that does not form any solid solution with the chalcogenide. In the protective layer, the composition ratio varies in its thickness direction, and the proportion of the compound is increased in the vicinity of an interface where it is contiguous to the recording layer. ZnS is exemplified for the chalcogenide while SiO.sub.2 is exemplified for the compound. The publication alleges that by increasing the proportion of SiO.sub.2 in the protective layer in the vicinity of an interface between the protective layer and the recording layer, it is possible to reduce the reaction between the recording layer and the protective layer, thereby preventing degradation of the recording layer and, consequently, achieving millions of recording and erasing cycles in a stable manner with no reflectance change. in Example 1 therein, a first dielectric layer (of 100 nm in thickness) comprising ZnS containing 20% of SiO.sub.2 is first formed on a substrate. A protective layer (of 20 nm in thickness) containing at least 90% of SiO.sub.2, a Te-Ge-Sb base recording layer, the aforesaid protective layer (of 20 nm in thickness), a second dielectric layer (of 200 nm in thickness) similar to the first dielectric layer, and an NiCr reflective layer (of 40 nm in thickness) are then provided on the first dielectric layer. In Example 2 therein, a protective layer having a gradient composition where the proportion of SiO.sub.2 becomes high (at least 90%) in the vicinity of the recording layer is used in place of the "dielectric layer plus protective layer" in Example 1.
However, it is now found that an optical recording medium prepared by the inventors following the examples in JP-A 2-177141 shows large jitters exceeding 13% already in an initial overwriting cycle, as can be seen from a comparative example given in the "EXAMPLE" in the present disclosure.
In connection with the optical recording medium prepared following the examples in JP-A 2-177141, it is also found that when the laser beam used for overwriting is modulated to such specific pulse patterns as depicted in FIGS. 6A and 6B, a substantial erase power margin narrows greatly upon repeated overwriting. In general, the erase power margin means an erase power range where the jitters are limited to a constant range. The erase power of a phase change type optical recording medium-driving device should be fixed at the optimum erase power according to the current standard. in actual applications, however, it varies from device to device depending on various causes such as variations in the properties of an associated optical pickup. The phase change type optical recording medium should thus have an erase power margin of width enough large to ensure interchangeability. In the optical recording medium prepared following the examples in JP-A 2-177141, however, it is found that when overwriting is repeated using laser beams according to the specific pulse patterns shown in FIGS. 6A and 6B, the upper and lower limits of the erase power margin as well as the optimum erase power are shifted to a lower power side. Consequently, the "substantial erase power margin" to be explained later becomes narrow. This in turn results in a reduction in the acceptable number of overwriting cycles.